1. Field of the Invention
This invention relates to thermostatic expansion valves for large-capacity refrigeration systems. It particularly relates to improvements in port balance of thermostatic expansion valves.
2. Review of the Prior Art
Thermostatic expansion valves, having an inlet and an outlet, a valve port therebetween, and a valve pin for selectively controlling the valve port, are well known in the art. Such valves generally comprise a compression spring for urging the valve pin or a plurality of push rods upwardly against a diaphragm of a diaphragm motor. On the upper or far side of the diaphragm is a chamber containing a sealed charge of a selected fluid which is exposed within a remote bulb to evaporated refrigerant as a temperature-sensing mechanism. In the lower or near side of the diaphragm is a chamber which is connected to the evaporated refrigerant as a pressure-sensing mechanism.
It has been known for some time that the pressure differential across the valve port produces an extraneous variable bias in the balance of forces that are intended to control the valve pin position. This bias adversely affects valve performance because of valve pin imbalance. The three basic forces intended to control valve pin position and, hence, flow through the valve are: (A) Bulb pressure, which is determined by the evaporator outlet temperature and is exerted on the top side of the valve operating diaphragm, produces a force tending to urge the valve in an opening direction as the remote bulb temperature increases;
(B) The evaporator pressure, which is exerted on the lower side of the operating diaphragm, opposes the bulb pressure and tends to urge the valve in the closing direction; and
(C) The spring force, which also tends to urge the valve in the closing direction, also opposes the bulb pressure.
All other forces affecting the position of the valve pin are extraneous. It is desirable, therefore, to minimize or eliminate them. The undesirable effect of valve pin imbalance is especially significant and troublesome in large-capacity valves with their necessarily large port areas. Several methods exist in the current state of the art to balance or offset the unbalance created by a pressure differential across the port. However, existing methods create other problems such as high manufacturing costs, high valve friction, partial imbalance, high internal leakage rates through the valve port or ports, and leakage from the valve inlet to the underside of the operating diaphragm. Such leakage of inlet pressure into the lower diaphragm compartment is particularly damaging to the performance of the expansion valve.
It is also highly desirable that the valve pin, through its entire stroke, operate uniformly in accordance with the pressures exerted on the diaphragm without being influenced by extraneous pressures and unpredictable unbalance or frictional forces.